IO INTRODUCTION 



that it was not long before the idea that the higher forms of 

 life arose in this way was discarded. 



Third Period. When the scientists of the day began to 

 study the microorganisms which Leeuwenhoek had dis- 

 covered, the theory of spontaneous generation was revived, 

 chiefly as the result of experiments by Needham (1713-1784), 

 and one of the fiercest battles known to science was fought 

 over this theory. It was very difficult to understand how it 

 was that putrescible fluids would spoil after they had been 

 heated if it were not true that the life which appeared in them 

 was spontaneously generated. It was generally believed that 

 the temperature of boiling water was sufficient to kill all forms 

 of life, and the wide distribution of microorganisms or their 

 spores in the air was not generally understood. One of the 

 first of the crucial experiments which pointed out the error 

 of Needham's experiments and led to the overthrow of the 

 theory of spontaneous generation was made by Abbe Lazzaro 

 Spallanzani (1729-1799), a fellow-countryman of Redi. In 

 1 77 7, he filled flasks with organic solutions, boiled them for 

 three quarters of an hour, and then sealed them and placed 

 them under conditions favorable for the growth of micro- 

 organisms without, however, having them develop. Spallan- 

 zani's critics objected to his experiments on the ground that 

 air had not been admitted, which they claimed was essential 

 to the life of these microorganisms. Franz Schulze, in 1836, 

 set aside the objections of these critics of Spallanzani by ar- 

 ranging a flask with two glass tubes bent at right angles so 



